This invention relates to a control circuit for a vehicle skid control system, and more particularly to a control circuit having an adaptive turn-off feature which terminates a brake-releasing brake control signal produced as an output from the control circuit and indicative of an imminent skid condition, thereby re-engaging the vehicle brakes so as to more effectively stop the vehicle, wherein the adaptive turn-off feature depends upon the coefficient of friction of the road surface being travelled by the vehicle and the degree to which the wheel speed decelerates in departing from the vehicle speed during the brake release cycle of the control circuit.
When the operator of a wheeled land vehicle desires to stop the vehicle under emergency braking conditions or under adverse road conditions, a realistic probability exists that the wheels of the vehicle may tend to lock up, thereby causing the vehicle to undergo an uncontrolled skid or a controlled skid, which could prevent the operator from stopping the vehicle within a safe distance. Under these circumstances, one factor indicative of an imminent skid condition is the relative relationship between vehicle wheel speed and vehicle speed. It is generally accepted by safety experts and professional land vehicle drivers that vehicle stability during an imminent skid condition can best be achieved by "pumping" of the brakes associated with the wheels of the vehicle in a manner providing for alternating braking and non-braking cycles. Vehicle skid control systems have been developed that operate on this principle of brake-pumping, wherein normal braking action as initiated by the operator of a wheeled land vehicle is selectively inhibited during operating conditions indicative of an imminent skid condition. Typically, such a vehicle skid control system utilizes wheel speed sensors to generate AC voltage signals that are proportional to vehicle wheel speed. The vehicle wheel speed signals are then processed through a frequency converter which generates a DC voltage proportional to vehicle wheel speed. Application of the vehicle brakes by the operator in a normal manner causes the wheels to decelerate. Should the deceleration rate of wheel speed as sensed by a deceleration rate detector equal or exceed a threshold level indicative of an impending wheel lock-up or skid condition, a brake control signal is produced from the control circuit of the skid control system to the vehicle brake actuator solenoid valve. Upon actuation of the brake actuator solenoid valve by the brake control signal, the fluid braking pressure to the vehicle wheels is relieved, thereby inhibiting the vehicle wheel braking action. The vehicle wheels then begin to spin-up or accelerate in speed. When the vehicle wheels spin-up to a speed approaching or approximating vehicle speed, the brake-inhibiting control signal is terminated, thereby de-energizing the brake actuator solenoid valve. This in turn restores line pressure in the vehicle braking system and re-engages the vehicle brakes. In effect, what is achieved by such a skid control system is an automatic "pumping" or "pulsing" of the vehicle brakes in a manner often recommended for controlled braking under adverse driving conditions. Vehicle skid control systems utilizing this automatic brake "pumping" principle are described in detail in my pending application Ser. No. 25,131, filed Apr. 2, 1970, and in U.S. Pat. Nos. 3,944,289 and 3,982,793, respectively, issued to me on Mar. 16, 1976 and Sept. 28, 1976.
A major problem encountered with vehicle skid control systems of the automatic brake pumping type heretofore developed is to determine the precise point during the brake inhibit cycle at which the wheel speed approximates the vehicle speed, i.e. the point at which the brake-inhibiting control signal produced by the control circuit should be terminated to enable the vehicle brakes to be reapplied. Although it is comparatively simple to determine that the vehicle wheels are decelerating at a rate indicative of an imminent skid condition, it is difficult, once the brakes have been disengaged by operation of the skid control system to determine the precise time at which the brakes should be re-applied because the vehicle speed, and therefore the degree to which the wheel speed is below the vehicle speed at a given instant are unknown. Typically, skid control systems attempt to approximate the vehicle speed and are designed to reapply the brakes when the wheel speed approaches the estimated vehicle speed. Under varying driving conditions, these types of skid control systems may not reapply the brakes soon enough because they fail to give proper consideration to either the coefficient of friction .mu. of the road surface on which the vehicle is travelling or the degree to which the wheel speed has departed from the vehicle speed during a wheel deceleration cycle producing a brake control signal relieving the brake pressure.
On high .mu. surfaces, such as dry concrete, a more rapid pulsing of the vehicle brakes is desirable, since the wheels tend to adhere to the braking surface because of the better traction and to spin-up at a faster rate once the brake inhibiting control signal is produced as an output from the control circuit of the skid control system. Similarly, the brakes of heavily loaded vehicles operating in a manner indicating an imminent skid condition should be pumped at a more rapid rate than lightly loaded vehicles. Conversely, on low .mu. surfaces, such as icy roads, the brakes should be pulsed relatively slower in order to bring the vehicles to a controlled stop in the shortest amount of time. However, on high .mu. surfaces, vehicle skid control systems heretofore developed may not reapply the brakes soon enough at the most optimum time intervals because the control circuit is designed more particularly to accommodate low .mu. surfaces, and, since the precise degree of wheel speed departure from vehicle speed is unknown, adjustments are not made in the operation of the skid control system to take the weight of the vehicle into account. Vehicle skid control systems of the type disclosed in my co-pending application Ser. No. 25,131, filed Apr. 2, 1970 referred to previously represent vehicle speed by a ramp signal produced in the control circuit. When the wheel speed signal in an accelerating mode (with a brake inhibit control signal being produced by the control circuit) crosses the ramp signal as the wheels spin-up, the brake inhibit signal of the system is terminated and the brakes are reapplied. However, the ramp signal is merely an estimate of true vehicle speed and may not be sufficiently accurate under all types of driving conditions to enable the skid control system to alternately disengage and reapply the brakes over the most optimum time intervals to accomplish such brake "pumping", especially when a large number of brake inhibit control signal cycles occur.